This application relates generally to error correction in disc drives and more particularly to a method and system for identifying and compensating for once per revolution timing errors.
Disc drives with written servo track information often experience the well-known phenomena of once per revolution (OPR) repeatable runout position errors resulting from a disc slip or misalignment. There are numerous causes for this disc slippage or misalignment, including thermal motor characteristics or high shock events. The disc slippage or misalignment may also be due to the servo track write process being performed on discs outside of the disc drive, such as with preformatted stamped discs. When the disc slips, or is misaligned, the disc drive head will be periodically out of position with respect to the center of the track leading to read/write errors.
Several different adaptive feedforward compensation (AFC) servo algorithms have been developed to compensate for these once per revolution repeatable runout position errors. For example, it is common to generate a servo Position Error Signal (PES) indicative of the position of the head of the disc drive with respect to the center of a selected track on a disc. The servo control system generates the PES from pre-recorded servo information stored in servo sectors on the disc drive. The servo system then uses the PES to generate a correction signal. The correction signal is provided to a power amplifier to control the amount of current through a voice coil motor actuator coil, in order to adjust the position of the head accordingly. The PES control deals satisfactorily with small OPR radial position errors.
However, disc slippage or misalignment also results in once per revolution (OPR) timing errors. These OPR timing errors result from disc misalignment in the timing direction on the disc rather than in the position direction on the disc. For example, as the position of the drive head is adjusted to correct OPR position errors, the path traveled by the head becomes less circular and more elliptical. Because the disc is divided into radial sectors in sectored servo disc drive systems and because of the adjusted path traveled by the disc drive head, OPR timing errors do occur. In a sectored servo disc drive system, the OPR timing errors present themselves as a spin speed tolerance or variation with a peak magnitude equal to the timing misalignment. To account for this misalignment, an inter-sector gap (ISG) field in the track formatting of the disc must be increased to adequately accommodate this spin speed tolerance. The inter-sector gap field is inserted between adjacent data fields on the back to account for fixed timing errors and OPR timing errors.
The size of the ISG field for each data field must be large enough to accommodate the cumulative error between servo fields in a sectored servo disc drive system with data fields between servo fields. Thus, the ISG field and format efficiency loss are magnified by the number of data fields between servo fields. For example, if there are five data fields between sequential servo fields and the resulting OPR timing error is 1%, then each ISG field separating each data field must be large enough to accommodate the 1% OPR timing error and any fixed timing errors. Such a conventional arrangement is shown in FIG. 6.
The servo sector 400 comprises servo fields 404 and a series of data fields 402. Separating each of the fields is an inter-sector gap field 406. The inter-sector gap field is inserted between data fields to account for fixed timing errors and OPR timing errors. In order to compensate for the OPR timing error, each inter-sector gap (ISG) field in the track format of the disc 108 (FIG. I) is sized in length to adequately accommodate the spin speed tolerance. In a sectored servo disc drive system with multiple data fields between servo fields, such as is shown in FIG. 6, the ISG field for each data field must be large enough to accommodate the cumulative error between consecutive servo fields. Thus, the ISG field and format efficiency loss are magnified by the number of data fields between servo fields. For example, still referring to FIG. 6, there are five data fields in the servo sector 400. Suppose the resulting OPR timing error is 1%. The size of each ISG field within the servo sector 400 must then be large enough to accommodate the 1% OPR timing error and any fixed timing errors. The size of the ISG fields together total a cumulative error of 5% between servo fields resulting in a 5% format efficiency loss, or a 5% data capacity loss. However, it should be understood that the actual OPR timing error varies from a minimum of zero to a maximum depending on the location of the head at a particular instance. Therefore, at many locations on the disc, the portions of the ISG field dedicated to correcting OPR timing errors are larger than is actually necessary to correct the OPR timing error and this is simply unused space.
Accordingly, there is a need for an improved disc drive system that identifies and compensates for actual OPR timing errors and eliminates the cumulative error between servo fields.
Against this backdrop the present invention has been developed. The present invention is a method for identifying and compensating for the cumulative effect of once per revolution (OPR) timing errors.
In one embodiment of the present invention, the nominal data field frequency is increased by one-half of the spin speed tolerance. An inter-sector runout (ISR) field is added between each of the data fields in a servo sector. Because the ISR fields will compensate for the OPR timing errors, the ISG fields in the disc format may then be reduced in size to compensate only for any fixed timing errors. The size of the ISR fields are rotationally variable from zero to the maximum OPR timing tolerance. The maximum OPR timing tolerance is the maximum amount of timing error tolerance that must be accommodated due to the elliptical shape induced by having an elliptical track. The ISR fields account for the actual OPR timing error rather than the maximum OPR timing error. Therefore, the format efficiency loss is reduced and more disk space is available for use.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.